Microporous materials are crystalline solids formed from three-dimensional networks of oxide tetrahedra that contain pores (less than 2 nm) and cages that allow for shape-selective ion exchange, separations, and catalysis. These materials often exhibit robust hydrothermal stability that allows their application under demanding process conditions such as fluidized catalytic cracking, exhaust gas emissions and treatment of toxic waste. Over 200 different microporous material frameworks have been identified, but of these less than 20 have been commercialized, and the market is dominated by only a few frameworks. Despite this seeming barrier to market entry, the demand to innovate in these materials remains high as there is often only a single framework and composition that deliver optimal performance in a given application. In recent years, microporous materials with pore diameters limited by 8-membered rings have received increased attention as they demonstrate good activity and hydrothermal stability for high demand applications such as the methanol-to-olefins (MTO) reaction (SAPO-34) and the reduction of NOX in emissions (SSZ-13).
Zeolite A (Linde Type A, framework code LTA) is one of the most used zeolites in separations, adsorption, and ion exchange. This structure contains large spherical cages (diameter ˜11.4 Å) that are connected in three dimensions by small 8-membered ring (8MR) windows with a diameter of 4.1 Å. LTA is normally synthesized in hydroxide media in the presence of sodium with Si/Al ˜1. By changing the cation, the limiting diameter of the 8MR windows can be tuned, creating the highly used series of adsorbents 3A (potassium form, 2.9 Å diameter), 4A (sodium form, 3.8 Å diameter) and 5A (calcium form, 4.4 Å diameter) that are used to selectively remove species such as water, NH3, SO2, CO2, H2S, C2H4, C2H6, C3H6 and other n-paraffins from gases and liquids. While LTA is used in vast quantities for the aforementioned applications, the low framework Si/Al ratio and subsequent poor hydrothermal stability limits its use under more demanding process conditions that are commonly found in catalytic applications. Strategies have been developed to increase the Si/Al up to 5.5 in hydroxide media using combinations of organic structure directing agents (OSDAs), and this material has been shown to be active for the MTO reaction.
Pure-silica LTA (ITQ-29) was first reported in 2004 and was synthesized in fluoride media using a combination of methylated julolidine (4-methyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3.2.1] quinolinium hydroxide (see FIG. 1(A))) and tetramethylammonium (TMA). Pure silica LTA showed an outstanding hydrothermal stability, and aluminum could also be introduced into the framework, making a material that showed activity for cracking as well as MTO. Pure-silica LTA has received considerable attention, especially for use in separations and as a membrane, since its hydrophobicity and small pore size show good discrimination for small molecules; it has also been studied as a low dielectric material. A method to synthesize germanosilicate LTA using a large polycyclic crown ether with the trade name Kryptofix 222 (see FIG. 1(B)) has been demonstrated. The material has 8MR openings that have 4.1 Å diameter and a spherical 3D network of 11.4 Å cavities. More recently, a method of preparing molecular sieves with LTA topologies have used triquaternary OSDAs, such as shown in FIG. 1(C).
In recent years there has been considerable interest in 8MR systems for catalysis and separations. Some of the most promising catalytic applications are the methanol to olefins (MTO) conversion and deNOx. Other 8MR materials of interest are LEV, CHA and AFX. It has been found that the cage size and connectivity are critical in determining the product distribution for these reactions in 8MR systems. LTA possesses a unique cage size and will likely exhibit unique catalytic performance. However, in order to produce an aluminosilicate material with the necessary silicon to aluminum ratio for the MTO reaction, a complicated OSDA is required, as shown in FIGS. 1(A-C). The nature of the organic makes it unlikely this material could be used in commercial production. The SDA-free syntheses of LTA can only be made at low Si/Al ratios and are less than optimal for catalysis.
The present invention is directed to addressing at least some of the shortcomings of the existing art.